Minerals Engineering, Vol. 7, Nos 2/3, pp. 209-221. 1994 0892-6875/94 $6.00+0.00 Printed in Great Britain © 1993 Pergamon Press Ltd

EFFECT OF DENSE MEDIUM PROPERTIES ON THE SEPARATION PERFORMANCE OF A DENSE MEDIUM CYCLONE

Y.B. HE and J.S. LASKOWSKI

Department of Mining and Mineral Process Engineering The University of British Columbia, B.C., Vancouver, Canada

* Author for communication (Received 4 May 1993; accepted 15 June 1993)

ABSTRACT

Extensive separation tests were carried out on a 6" dense medium cyclone loop with the use of density tracers to study the effects of medium stability and rheology on the separation of fine particles. The medium properties were modified by changing the magnetite particle size and medium density. Four magnetite samples with RRB size modulus d63.2 ranging from 4. 7 to 33 Izm were used, and the medium densities were varied fi'om 1.2 to 1.7 s.g. It was found that, while the separation e.ffTciency and cutpoint shift for coarse particles (> 2. 0 ram) were mainly determined by the medium stability, the separation performance of fine particles (< O. 5 ram) was more sensitive to the change in medium rheology.

Keywords Dense medium, magnetite, magnetite suspension, gravity separation, dense medium cyclone, stability, rheology, separation efficiency.

~TRODUCTION

Medium rheology and stability play an important role in dense medium cyclone (DMC) separation. The factors that control the medium properties are particle size distribution, particle shape, medium density (solid content), and medium contamination. Medium rheology affects the separation efficiency of fine particles by reducing the particle movement in the medium. The effect of magnetite particle size was studied by Fourie et al [1] and Chedgy et al [2]. They found that by using progressively finer magnetite, the separation efficiency was improved due to increased medium stability. Fourie et al [1] recommended that, for the sharp separation of coal, at least 50 percent of the magnetite be finer than 10 microns. An opposite trend was observed by Collins et al [3] when working at high medium densities; they observed that the sharpness of separation declined when a finer grind ferrosilicon was utilized. They advocated the use of spherical medium particles to reduce viscosity and showed that the separation efficiency was better when atomized spherical ferrosilicon was used. Several researchers [3, 4] have found that the effect of medium density on separation efficiency was insignificant within a limited density range. Davis and Napier-Murm [6], in studying the effect of medium contamination on cyclone performance, showed that the clay contamination significantly reduced the separation efficiency due to an increased effect of medium theology.

Separation efficiency is also affected by medium stability. The formation of density zones in the cyclone leads to the misplacement of feed particles, reducing the separation efficiency. Davis [7], in a full scale

209

210 Y.B. HE and J. S. LASKOWSKI

DMC test, noted that a higher density differential led to a longer retention time of near-density materials. The cyclone became periodically overloaded resulting in a breakdown of the flow pattern and surging. Collins et al [8] recommended that for a satisfactory separation the density differential be maintained within 0.2 and 0.5 s.g.

The separation cutpoint, another performance parameter in gravity separation, is more directly affected by medium stability; the effect of medium rheology on the separation outpoint has been interpreted in terms of its influence on medium stability. At a constant medium density, the outpoint decreased when the medium was stabilized by the addition of clay [6] or by the reduction of magnetite particle size [9]; the separation cutpoint was influenced by medium viscosity through the changing of the underflow medium density since a close correlation between cutpoint and underflow medium density was observed [6].

Studies on the effect of medium properties on the DMC performance are complicated due to the contradicting influences of medium rheology and medium stability which, in the dynamic DMC separation process, is also a function of cyclone operating conditions. In addition, feed particle size also plays a very important role in the relationships between the cyclone performance and the medium properties. Although efforts have been made by many researchers to understand such relationships, many of the derived conclusions revealed only different parts of the entire picture. In the present investigation, extensive tests have been carried out to determine the separation responses of different feed particle sizes to the effect of medium properties by using synthetic density tracers. The medium properties have been modified by changing the magnetite particle size distribution and magnetite content in the medium.

EXPERIMENTAL

Materials

Four magnetite samples were used to prepare the dense media. The magnetite samples, which covered a broad range of particle size distributions, were well represented by the Rosin-Rammler-Bennet distribu- tion; and their size and distribution moduli are given in Table 1. Mag#2 was obtained by grinding Mag#1, a commercial magnetite sample provided by Craigmont Mines, B.C., Canada, in a ball mill; Mag#3 was prepared by classifying Mag#1 in a classifying cyclone circuit to eliminate the fines. Mag#4 was the micronized-magnetite (70%<5 I.tm) kindly provided by the Department of Energy, Pittsburgh. The medium density was varied from 1.20 to 1.70 s.g.

The colour-coded density tracers obtained from the Partition Enterprises Ltd., Australia, were used as cyclone feed. The following size fractions of the density tracers were utilized: 4.0x2.0, 1.0x0.71, and 0.5x0.355 ram.

TABLE 1 RRB size and distribution moduli of the magnetite samples

Sample d63.2(I.tm) m

Mag#1 30.5 3.2

Mag#2 18.0 1.6

Mag#3 33.0 4.1

Mag#4 4.3 1.9

Dense medium cyclone 211

Techniques

The separation tests were conducted on a pilot scale DMC loop. The schematic illustration of the loop is given in Figure 1. The 6" cyclone (model D6B-12*-S287), obtained from Krebs Engineers International, California, was gravity-fed at an inlet pressure of 60.6" liquid column (10xD). The cyclone vortex finder and spigot diameters were 2.5" and 2.0", respectively.

Headbox

r

6" Cyclone

Siev . . . . . . . . I . . . . . . . . . I

Fig. 1

Sump

Pump

A schematic diagram of the 6" dense medium cyclone loop

During the separation tests, the density tracers of different densities were always kept separate. Each time, only one density fraction was introduced into the cyclone loop from the head box; the minimum weight for each density fraction was about 100 grams. The tracer particles reporting to overflow and underflow were recovered on the screens mounted in the sampling boxes, while the carrying medium was recycled. The tracer particles in the two products were washed, dried and weighed separately. From the weights, a partition number was calculated. The whole process was repeated with different density fractions to get enough data points for constructing a partition curve. To ensure accuracy, duplicate data points especially around the separation cutpoint were produced. The densities and flow rates of the overflow and underflow media were monitored throughout the testing. From these data, the overflow to underflow flowrate ratios and density differential were calculated.

Rheological measurements were carried out using a HAAKE Rotovisco RV20 rheometer. A sensor system (ESSP) specially designed and built for the rheological measurements on rapidly settling suspensions [10] was employed. The shear rate was programmed to increase linearly from 0 to 300 s "1 in one minute to obtain complete flow curves.

212 Y.B. HE and J. S. LASKOWSKI

RESULTS AND DISCUSSION

Cyclone Operating Conditions

The DMC separation performance is affected by both medium properties and cyclone operating conditions. To determine the effect of medium properties, the cyclone operating conditions need to be optimized and kept constant throughout the tests.

As we have previously reported [ 11], the cyclone overflow to underflow flowrate ratio (O/U ratio) is one of the most fundamental operating variables affecting DMC performance; the spigot and vortex diameters only affect the DMC performance indirectly through changing the O/U ratio. The fundamental property of the O/U ratio is reflected by its close correlation with medium stability, separation efficiency, and outpoint shift. At constant medium density and composition, as revealed by Figure 2, increased O/U ratio resulted in a greater differential between the cyclone underflow and overflow indicating the influence of the cyclone operating conditions on medium stability. Under idealized separation conditions when feed-to- medium ratio approached zero (Figure 3), a higher O/U ratio resulted in a lower Ep value; the outpoint shift (the difference between separation outpoint and medium density) exhibited a well defined linear relationship with the O/U ratio on a semi-log plot.

0.6

0.5 A

O)

'W 0.4

j O.3

"0

-->' 0.2 OD C 0 a

0.1

0 0.1 100

Medium density 1.36 s.g. /

/ Mag#2 Vortm( diameter ~

_ ~ ~ Vortex diameter e " ~ r ~ " " ~ - 1 .S', 2.5", & 3.0"

I

1 10

O/U ratio

Fig.2 The effect of the overflow to underflow flowrate ratio on the density differential for magnetites #2 and #4

The results in Figure 3 also indicate the working range of the O/U ratio under idealized separation conditions. It seems beneficial to run the DMC at a higher O/U ratio to lower the Ep values. Nevertheless, a very high O/U ratio with real feed would cause intermittent cyclone jamming and roping in the separation process due to a reduced underflow discharge capacity and an elevated density differential which will further increase the retention time of near-density materials. An optimum O/U ratio of around 2.0 was recommended for coal preparation [11], which corresponds approximately to the volumetric split ratio of coal feed to overflow and underflow. In this investigation, the O/U ratio was kept at around 1.8 by selecting a combination of a vortex finder diameter of 2.5" and a spigot diameter of 2.0". The O/U ratio is mainly a function of medium flowrate and the cyclone vortex finder and spigot diameters. With a fixed cyclone configuration and inlet pressure, the O/lJ ratio was found to be practically independent of medium properties. As shown in Figure 4, it remained around 1.8 with various magnetite samples over the entire range of tested medium densities.

Dense medium cyclone 213

6)

¢-

( -

O Q . - i O

- i

t¢ > Q . u.I

0.30

0.20

0.10

0

-0.10 0.12

0.10 -

0 .08 -

0 . 06 -

0 . 04 -

0 . 02 -

00.1

Vortex=2.5" medium dertsityffi 1.36 tracerffi 1.0x.71 mm

L I o MagdP2 I • Mag#4 I

i

1 lO OAJ ratio

100

Fig.3 The effect of the overflow to underflow flowrate ratio on separation efficiency and cutpoint shift for magnetites//2 and #4

A

m

(9

/: O LL

3.5

2.5 3

O

o 1 :Z)

%,, o o o o o o o o o 11

. . . . . . . .

Oritice:2.5"-2.0"

. Mag#1 o Mag#2 A Magi3

M a g # 4

. . . . .'5 ' ' ' 1.1 1.2 1.3 1.4 1 1.6 1.7 1.8

Medium density (s.g.)

Fig.4 The effect of medium density and magnetite particle size distribution on the medium flowrate and the overflow to underflow flowrate ratio

1.9

Medium flowrate is another important operating variable and should also be maintained constant. The results in Figure 4 indicate that the medium flowrate at a constant inlet pressure of 60.6" liquid column was almost unaffected by the changes in medium density over the tested range and in magnetite particle size distribution (with the only exception of coarse Mag#3 dense medium which exhibited a higher flowrate).

214 Y . B . HE and J. S, LASKOWSKI

With fixed circuit and cyclone configurations and, more importantly, with a constant O/U ratio of 1.8 and a constant medium flowrate, any change in the DMC performance can be related to the change in the medium properties.

Medium Stability and Medium Rheology

Stability and rheology are the two principal medium properties affecting DMC performance. Like coal particles, the magnetite particles in a DMC are also subjected to the effect of centrifugal acceleration and experience classification which is reflected by the disparity between the cyclone overflow and underflow medium densities. The degree of classification, as Figure 5 shows, is a function of magnetite particle size and medium density. The Mag#3 dense medium was extremely unstable with a highly concentrated underflow and a very diluted overflow. As will be shown later, the extremely low stability was responsible for the poor DMC performance with the Mag#3 dense medium. The micronized-magnetite (Mag#4) dense medium, by contrast, was very stable; the overflow and underflow medium densities were almost identical.

2.2

"~ 2

"~ 1.8

-8 1.6

1.4

1.2 0

/ • • • Mag#3 underliow/•

~|o°O • ° ° °

• ~ v Mag#1 underllow v .Mag#2 undedlow •

'V~ 'V ~ • Mag#4 undedlow " Mag.f/'4 overflow ~,

, , v • • • ~ ' o ~ ' Mag#2 overflow o v • A,./ r-t ,c...7 V v • •" . ~ e o~ ,~ Mag.f/'l overflow '7

v v . . " - ' ~ ° V V M a g # 3 oved low °

, _ / / o v o~ .~o ~7 ~ ~7 ~ ~

/ . , . , ,~ ~ v _ o o ° ° o

/ " I o 0 ~ L K k n 1.2 1.4 1.6 1.8 2

Feed medium density (s.g.)

Fig.5 The effect of magnetite particle size distribution and medium density on the overflow and underflow medium densities

2.2

The density differential between cyclone under flow and overflow characterizes the medium stability. It is affected both by cyclone operating conditions (Figure 2) and by medium properties (Figure 6) and, therefore, it is considered to be a parameter describing the dynamic stability of the medium. For all tested magnetites, the density differentials initially increased with medium density but then declined at higher medium densities (Figure 6). This was apparently duo to the greater influence of medium rheology. Otherwise, the density differential should have followed the initial trend and continued to increase. It can also be seen from Figure 6 that, at the same medium density, the density differential is higher with coarser magnetite particles.

A detailed discussion of magnetite dense medium rheology and its effect on separation efficiency will be given in another paper [12]. Extensive rheological measurements revealed that magnetite suspensions exhibit shear thinning properties with yield stress; the rheologieal flow curves for magnetite suspensions

D e n s e m e d i u m c y c l o n e 215

can be best described by the Casson equation "c=('%l/2+(vlcy)t/2):. While the Casson viscosity (or Bingham plastic viscosity) was found not to be very sensitive to the changes in magnetite content and particle size distribution in the suspension (Figure 7), the Casson yield stress responded to these changes in a logical and well-defined manner (Figure 8). The Casson viscosity and Casson yield stress are two independent variables that define the theology of magnetite suspensions. As shown in Figures 7 and 8, the Casson yield stress reflects the changes in magnetite medium rheology while Casson viscosity is only a subordinate parameter. The results of Figure 8 reveal that a decrease in particle size, or an increase in medium density, substantially changed medium rheology by increasing the Casson yield stress.

0.3

0.25

O )

m,~ 0.2 .~_ r - - q )

.~ 0.15

"lo

w 0.1 ¢- ID 0

0.05

f

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Feed medium dense / ( s . g . )

1.1

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

.9o.1

Fig.6 Medium stability as a function of magnetite particle size distribution and medium density

7

A 6 m. t.s g ~ ' 4 3

g ~e 2

O 1

0 1.1

Temp.=19 C A

! !

! i

/ Mag#4 t A !

! I

! i

i / O/ O / , / .8

L - ? : . . . . . ~..-""" \ Mag#1 - ~ ,,. / g . . . . \

k : : ~ 8- : ~ - - 0 . ° 0. ~ - 7~-. ~ ~ \ ~ ' " - A "O" . . . . 7:) ," 0 - ' - . . . . ~ '¢

" ' ~ . /" 0 / "0" - ~,

A " . . . A A . / '

'2 .......................... ~ Hag#2 A

I I I I I I I I I L I

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 Medium density (s.g)

Fig.7 The effect of magnetite particle size distribution and medium density on the Casson viscosity

216 Y . B . HE and J. S. LASKOWSKI

Fig.8

100

10

I1. u) U) ® 1 t , .

"o (I)

, u

> , 0.1 t -

O (n u)

(3 0.01

~ ~ Mag#l

D~.~ ~ temp. =19 °c

0 . ~ 1 i t I J I t i I i ~ i

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3

Medium density (s.g) Casson yield stress as a function of medium density and magnetite particle size distribution

Separation Efficiency

The magnetite dense medium properties affect DMC separation efficiency. As seen from Figure 9, the increased medium density, and the associated change in medium rheology, resulted in a general increase in Ep values. With finer feed particles, the Ep values increased more significantly especially over the high medium density range. A small increase in Ep values for the coarse feed particles (4x2 mm) in Figure 9 may suggest that the effect of medium rheology on the separation of coarse particles is insignificant and that such particles are more susceptible to the effect of medium stability. Figure 9 confirms this presumption; in comparison with Mag#1, lower Ep values were obtained with the use of more stable Mag#2 dense medium for the coarse feed particles (4x2 mm). On the other hand, the dramatic increase in Ep values for the fine feed particles (0.5x0.355 nun) implies that the separation of such particles is more sensitive to the effect of medium rheology. As a result, lower Ep values were obtained for the fine feed particles when using a less viscous dense medium prepared from Mag#1. Such an analysis leads to the conclusion that the significance of the effect of medium stability and medium rheology is determined not only by the medium property itself but also by the feed particle size.

The medium rheology and medium stability have counteracting effects on the DMC separation efficiency. As shown in Figure I0, the Ep values with Mag#1, Mag#2, and Mag#3 tend to increase with increasing medium density, and the most dramatic increase in Ep value was observed with micronized-magnetite Mag#4 when medium density exceeds 1.5 s.g. This obviously resulted from the change in medium rheology. The density differentials for the three magnetite dense media were all confined within 0.5 s.g. (Figure 6); the adverse effect of medium stability on separation efficiency, according to Collins et al [8], was negligible. The medium rheology, as characterized by the Casson yield stress in Figure 8, became the dominating parameter in affecting the separation efficiency. The best separation efficiency was obtained with the Mag#1 dense medium due to its low Casson yield stress over the entire tested range of medium density. At low medium densities (< 1.5 s.g.), the effect of medium theology is not very important. As a result, the difference among the Ep values obtained with Mag#1 and Mag#2 dense media were insignificant. Over the high medium density range, however, the difference grew due to an increasing effect of medium rheology, and the advantage of using coarser Mag#1 dense medium was evident.

Dense medium cyclone 21"/

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

Fig.9

0.08

Mag#1

• tmcer .Sx.355mm

o tracer 1x.71mm

tracer 4x2mm

o

o

• o ° o o

o o

o • • o • •

J J . L L 1

• 2 1.3 1.4 1.5 1.6 1.7

Mag#2 ° e

• • ~ o

e e e o

• o

o

o o

o ° o

o •

• A • • A

A •

_ _ ~ _ _ L 1 J

.8 1.3 1.4 1.5 1.6

O o o o

o

• &

1.7 1.8

Medium density (s.g.) The effect of medium density and tracer particle size on separation efficiency

for magnetites #1 and #2

O m m

EL U.I

0.07

0.06

0.05

0 . 0 4 -

0 . 0 3 -

0.02 ~-

0.01 1.1

Tracer 1.x.71mm, orifices 2.5"-2.0"

• n

~ D - M a g # 1 A "

I I I I I I I

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

Fig. 10

Medium density (s.g.) The effect of magnetite particle size distribution and medium density

on separation efficiency for lx0.71 nun tracer particles

It must bo pointed out that the top particle size is obviously not as important as the particle size distribution. The top particle sizes of Mag#1 and Mag#3 were almost identical (Mag#3 was obtained by M E 7 - 2 / 3 - 4 3

218 Y . B . HE and J. S. LASKOWSKI

removing fines from Mag#1). It was observed that the effect of medium rheology on separation efficiency diminished when Mag#3 was utilized. The effect of medium stability, however, became dominant. As seen from Figure 6, the density differential for the Mag#3 dense medium (0.8 to 1.0 s.g.) was much higher than the recommended value of 0.2 to 0.5 s.g. [8]. The extremely low stability exerted a deleterious effect on the DMC separation efficiency; very high Ep values were observed over the low medium density range. With increasing medium density, the medium stability improved and a better separation efficiency was achieved.

According to the results of Figure 10, the best separation efficiency was achieved by using the Mag#1 or Mag#2 dense media over the low medium density range (< 1.5 s.g.). Clearly, an extremely low stability for the Mag#3 dense medium (Figure 6), and a very high Casson yield stress for the Mag#4 dense medium (Figure 8) contributed to the low separation effieiencies with these two magnetite samples over low and high density ranges, respectively. In the DMC separation of fine coal, a higher centrifugal acceleration and finer magnetite dense medium is advocated [13]. According to Figure 6, Mag#1 is close to the upper limit of particle size distribution in terms of medium stability. Any attempt to use higher centrifugal acceleration for such a magnetite would cause an excessive medium segregation. This might offset any beneficial gain from an increased centrifugal acceleration and the separation efficiency might deteriorate. The Mag#2 dense medium characterized by an intermediate particle size distribution, on the other hand, is a compromise that maintains a higher medium stability without imparting a significant effect of medium theology on DMC separation. Obviously, a higher centrifugal acceleration is acceptable in this case and may result in a much better separation efficiency.

Chedgy et al [4] found that, when using commercial magnetite at low medium densities, the separation efficiency deteriorated when the cyclone inlet pressure was increased. Obviously, the medium stability was a dominant factor; the deleterious effect of an elevated medium classification at higher inlet pressure more than offset the beneficial gain from an increased centrifugal acceleration. An opposite trend was observed by Klima and Killmeyer [ 13] when using mieronized-magnetite (90 % < 5 tim) in the separation of fine coal; a sharper separation was obtained at a higher cyclone inlet pressure. The highly stable medium allowed the use of an elevated centrifugal acceleration without experiencing an excessive classification. The results of Figure 10 may suggest that the use of micronized-magnetite (Mag#4) over the high medium density range be prohibitive. At low medium densities, however, the advantage of using the micronized-magnetite may prevail especially under high cyclone inlet pressures.

Cutpoint and Cutpoint Shift

The separation cutpoint, 850, and separation efficiency, Ep, are the two most important performance parameters in dense medium separation. Fluctuation in 850 results in the misplacement of particles and, consequently, reduces the overall separation efficiency.

Under normal operating conditions, 850 is higher than the feed medium density and the 850 of finer feed particles is always higher than that of coarse feed particles. The cutpoint is determined not only by the medium density and feed particle size but also by the medium properties. As shown in Figure 11, the cutpoint for the 1.0x0.71 mm density tracers with the highly stable Mag#4 dense medium was almost identical with the medium density; the corresponding cutpoint shift was close to zero. As the medium stability decreased when Mag#1 and Mag#2 were utilized, the 850's started to deviate from the medium density, and higher outpoint shifts were observed. With the extremely unstable Mag#3 dense medium, it became very difficult to accurately control 850 by adjusting the feed medium density alone since a slight variation in feed medium density resulted in a dramatic 850 fluctuation.

The cutpoint shift is apparently more closely related to the medium stability which, in the above tests, was altered by changing the magnetite particle size and medium density. Davis and Napier-Munn [6], who stabilized the medium by increasing the amount of fine clay contaminant, observed a similar phenomenon: the outpoint shift decreased as the medium stability increased; with a very high level of clay contamination, the cutpoint shift approached zero. The medium theology was believed to influence the cutpoint shift indirectly through changing the medium stability.

Dense medium cyclone 219

0.4

Eg

.¢2_ ¢ -

c- o m

o o . ,O=a

O

0.3

0.2

0.1

0

-0.1

-0.2 1.1

Tracer 1.x.71mm, odrces 2.5'-2.0" Mag#3

.//~ • ^ ~ _ ~ Mag#1

^ ~ £ ~ ~ - ~ ~ - - ~ Mag#2

~ e - " " - • • Mag#4

L L L .L . I . _ _ ~

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Medium Density (s.g.)

Fig. 11 The effect of magnetite particle size distribution and medium density on cutpoint shift for lx0.71 mm tracer particles

The effects of medium stability and theology on cutpoint shift are also interrelated with feed particle size. As a result, two opposite trends in cutpoint shift for fine and coarse feed particles are observed in Figure 12. The outpoint shift of coarse feed particles (4.0x2.0 mm) is mainly affected by medium stability; improving the medium stability by increasing either medium density (Figure 12) or medium contamination [6] will result in a cutpoint shift decrease. The shape of the cutpoint shift versus medium density curve for the 4.0x2.0 rnm feed particles in Figure 12 is similar to that of density differential versus medium density curve in Figure 6. The cutpoint shift of fine feed particles (0.5x0.335 mm), on the other hand, is more affected by the medium theology; a monotonic increase in cutpoint shift with medium density was observed in Figure 12. The shape of the cutpoint shift versus medium density curve for the 0.5x0.355 mm feed particles in Figure 12 is similar to that of Casson yield stress versus medium density curve in Figure 8.

In general, it can be derived from the results in Figures 9 and 12 that, while the separation of coarse particles ( > 2 . 0 ram) is more affected by medium stability, the separation of fine particles (<0 .5 nun) is mainly determined by medium rheology.

CONCLUSIONS

The effects of medium stability and rheology on DMC performance are interrelated with feed particle size. The separation efficiency and cutpoint shift for coarse particles (> 2.0 nun) are mainly determined by the medium stability. A higher separation efficiency and a lower cutpoint shift can be achieved with finer magnetite. On the other hand, the separation efficiency and cutpoint shift for fine particles ( < 0.5 mm) are more affected by medium rheology; an increased effect of the medium rheology results in a lower separation efficiency and a higher cutpoint shift.

Medium stability and rheology have different effects on the performance of DMC. As a result, two opposite trends are observed in the relationship between separation efficiency and medium density. With fine magnetite, the effect of medium rheology plays a major role: the separation efficiency deteriorates

2 2 0 Y . B . H e a n d J. S. LASKOWSKI

when the medium density is increased. With coarse magnetite, however, the effect of medium stability becomes more important; an increase in medium density results in a better separation efficiency. The medium stability and theology are determined by the magnetite particle size distribution; an appropriate control of the magnetite particle size distribution is more important than controlling its top particle size.

0.15

A

o. = 0.1

.¢::

o.os 0

Fig. 12

Mag#2

l I I I I

1.2 1.3 1.4 1.5 1.6 1.7

"1 oi ii o o o-~

I t I I I

1.3

0.2

0.15

0.1

¢p.

¢ -

E 9

o.o o

0

1.4 1.5 1.6 1.7 1.8

Medium density (s.g.)

The effect of medium density and magnetite particle size distribution on cutpoint shift for three different sizes of tracers

At low medium densities (< 1.5 s.g.), the best separation efficiency is achieved by using magnetite with intermediate particle size distributions (d63.2 = 15-20 pro). This compromise allows a higher medium stability to be achieved without any significant change in medium theology. At high medium densities (> 1.5 s.g.), the effect of medium rheology becomes dominant; it is beneficial in this density range to use coarser magnetite to improve separation efficiency.

Our results indicate that in a three-product dense-medium separation it would be beneficial to use freer magnetite medium in a low-density circuit and coarser magnetite in the secondary high- density circuit.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the financial support of the Science Council of British Columbia.

I .

2.

.

REFERENCES

Fourie, P.J.F., van der Walt, P.J. & Falcon, L.M., The Beneficiation of Fine Coal by Dense- medium Cyclone, J. S. Afri. Inst. Min. MetalL, 357-361 (1980). Chedgy, D.G., Watters, L.A. & Higgins, S.T., Heavy Media Cyclone Separations at Ultralow Specific Gravity, Prec. 10th Int. Coal Preparation Congr., Edmonton, Canada, vol. 1, 60-79 (1986). Collins, B., Napier-Munn, T.J. & Sciarone, M., The Production, Properties, and Selection of Ferrosilicon Powders for Heavy-Medium Separation, J. S. Afri. Inst. Min. Metall., 103-115 (1974).

Dense medium cyclone 221

o

5.

6.

.

8.

9.

10.

11.

12.

13.

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